1. Field of the Invention
The present invention relates to the field of surface acoustic wave (SAW) devices, and specifically to improvements to in their design and manufacturing which then provides additional applications for use.
2. Description of the Prior Art
Acoustic wave sensors use a detection arrangement that is based on perturbations to mechanical or acoustic waves. As an acoustic wave propagates through or on the surface of the acoustive wave sensor material, any changes to the physical or chemical characteristics of the wave path may affect the velocity and/or amplitude of the acoustic wave. These changes may be correlated to the corresponding physical, chemical, or biological quantities being measured to provide sensing.
There may be various biological and chemical sensors, using fiber optics, chemical interactions, and various fluorescence approaches. Such sensors may, however, have various weaknesses, such as, for example, low sensitivity, selectivity, or an inability to be hybridized or integrated into sensing chip technology. Acoustic wave (AW) sensors, however, may be better suited for use in biological and chemical detection. As discussed in D. S. Ballantine, R. M. White, S. J. Martin, A. J. Ricco, E. T. Zellers, G. C. Frye, H. Wohltjen, “Acoustic Wave Sensor—Theory, Design, and Physico-Chemical Applications”, Academic Press, (1997), acoustic wave sensors may use piezoelectric crystals, which may allow transduction between electrical and acoustic energies. The AW sensor may use piezoelectric material to convert a high frequency signal into an acoustic wave, and the higher frequency may enable the sensor to be more sensitive to surface perturbations.
Piezoelectric materials used for acoustic wave sensors may include quartz (SiO2), lithium niobate (LiNbO3), zinc oxide (ZnO), and others. Each of these materials may possess specific advantages and disadvantages, which may relate to, for example, cost, temperature dependence, attenuation, and propagation velocity. Such materials may, however, have varying transverse acoustic wave velocities, low electromechanical coupling coefficients, non-linear temperature coefficients, and may react chemically with the environment. (See the background information in C. Caliendo, G. Saggio, P. Veradi, E. Verona, “Piezoelectric AlN Film for SAW Device Applications”, Proc. IEEE Ultrasonic Symp., 249-252, (1992) and K. Kaya, Y. Kanno, I. Takahashi, Y. Shibata, T. Hirai, “Synthesis of AlN Thin Films on Sapphire Substrates by
Chemical Vapor Deposition of AlCl.sub.3—NH.sub.3 Systems and Surface Acoustic Wave Properties”, Jpn. J. Appl. Phys. Vol. 35, 2782-2787, (1996) and G. Carlotti et al., “The Elastic Constants of Sputtered AlN Films”, Proc. IEEE Ultrasonic Symp., 353, (1992)).
Previously, creation of SAW devices has been complicated and, in the case of CMOS fabrication, it has been unworkable as the chip would be destroyed by the temperatures required to integrate the SAW device.
One of the major distortions in the transfer characteristics of SAW devices occurs due to the angular spreading of the surface wave. This spreading occurs due to the finite aperture of the conventional SAW IDTs (Interdigital Transducers). The finite aperture causes a curved wavefront rather than the desired flat one which in turn generates increased insertion loss, passband distortion and reduction of out of band rejection [1]. FIG. 1 depicts this phenomenon. The diffraction can be decreased by employing wider acoustic apertures. An improvement of up to 30 dB in out of band rejection was realized by increasing the IDT finger apodization overlap as in [2]. This compensation requires careful adjustment of both the amplitude and phase of the SAW radiated by each IDT finger and is achieved by using IDTs with split-electrode geometries [3].
In the SAW literature, focusing interdigital transducers (FIDT) have been extensively utilized in devices such as convolvers [4], storage correlators [5], time-Fourier transformers [6], and radio frequency (RF) channelizers [7]. All of these devices employed the FIDTs to generate high intensity acoustic fields. The focusing phenomena of SAW fields have also drawn recent interests. Wu et al. [8] gave a detailed account of analysis and design of focused IDTs. In their work, they adopted the exact angular spectrum of plane wave theory (ASoW) to calculate amplitude fields of FIDTs on Y-Z Lithium Niobate (LiNbO3) substrates. Qiao et al. [9] on the other hand, applied rigorous vector field theory of surface excitation on the crysTal to investigate the focusing phenomena. As concluded, the theoretical results demonstrate that anisotropy of the medium has a great impact on the focusing properties of the acoustic beams, such as focal length and symmetrical distributions near the focus [9]. Although FIDTs have been proposed extensively in ultrasonic and acoustic literature for many years, most of them focused on curved IDTs. The most prominent architecture that was investigated in this realm is circular-arc interdigital transducer (CIDT). In their work Fang et al. [10] demonstrated the focusing characteristics of a CIDT on Y-Z Lithium Niobate (LiNbO3) using the angular spectrum theory and experimental velocity data. They concluded that the use of CIDT on Y-Z LiNbO3 can provide a very narrow and long focused acoustic field rather than a localized focal point. They also suggested that CIDT structure does not construct efficient focusing for very highly anisotropic materials. Conventional circular arc structures were also employed in [8] and [9]. Kharusi et al. proposed the FIDT shape as the wave surface [11]. They reported that its focusing ability is better than that of the FIDTs with circular arc shape. The wave surface is the locus of points tracked by the end of the energy velocity vector which is drawn from a fixed original point. The idea of using the wave surface as a design method gives rise to a variety of possibilities to be employed for different piezoelectric materials depending on their crystal isotropy and c-axis orientations.